The present invention relates to increasing heat transfer performance of a ventilation chimney in the rotor of a dynamoelectric machine. Specifically, the invention relates to turbulating the surface of a ventilation chimney in a rotor to increase the heat transfer performance.
The rotors in large gas cooled dynamoelectric machines have a rotor body which is typically made from a machined high-strength solid iron forging. Axially extending radial slots are machined into the outer periphery of the rotor body at specific circumferential locations to accommodate the rotor winding. The rotor winding in this type of machine typically consists of a number of complete coils, each having many field turns of copper conductors. The coils are seated in the radial slots in a concentric pattern with, for example, two such concentric patterns in a two-pole rotor. The coils are supported in the rotor body slots against centrifugal forces by wedges that bear against machined dovetail surfaces in each slot. The regions of the rotor winding coils that extend beyond the ends of the main rotor body are called “end windings” and are supported against centrifugal forces by high strength steel retaining rings. The section of the rotor shaft forging which is disposed underneath the rotor end windings is referred to as the spindle. For ease of reference and explanation herein-below, the rotor winding can be characterized as having a central radial flow or diagonal flow region between the end winding discharge chimneys, a rotor end winding region that extends beyond the pole face, radially spaced from the rotor spindle, and a slot end region which contains the radial flow ventilation or discharge chimneys. The slot end region is located between the central radial flow region and the rotor end winding region.
The design of large turbo-electric or dynamoelectric machinery requires high power density in the stator and rotor windings. As ratings increase, both specific loading of the windings (i.e., current carried by a given cross section) and the distance to a heat sink such as a cooler (or heat exchanger) also increase. Additional cooling technology can be employed to carry heat out of the parts of the generator.
Direct cooling of the rotor windings is a well-established practice in electric machinery design. The cooling medium, typically hydrogen gas or air, is introduced directly to the winding in several ways. The gas may enter the rotor through subslots cut axially into the rotor forging. and exhaust through radial ducts in the copper. The pumping action caused by rotation of the rotor and the heating of the gas pulls gas through the subslot and out the radial ducts. Alternatively, gas may be scooped out of the gap at the rotating surface of the rotor and may follow a diagonal or radial-axial path through the copper winding. The gas exhausts once again at the rotor surface without need for a subslot. These two strategies cool the windings in the rotor body.
Rotor end turns may require additional cooling. One established method for this is to place one or more longitudinal grooves in the copper turn. The groove connects to an outlet at or near the rotor surface that will pull gas through the groove. The outlet can be a radially directed duct at the end of the rotor body, or the grooves can lead to a vent slot in the tooth or pole of the rotor body. In general, the retaining ring that mechanically supports the end turns is not penetrated. The end turn grooving strategy can be used with any type of rotor body cooling, either radial, radial-axial, or gap-pickup. End turn cooling grooves can also exhaust to a radial ventilation or discharge chimney.
To exhaust the end section gases, the discharge or ventilation chimney is located in the outermost axial position of the rotor body, where it receives no additional cooling from the radial or diagonal flow ducts in the center body section. The discharge chimney is typically the hottest section in the rotor, limiting power output since electrical insulation temperature limits should not be exceeded.
Because of the large number of grooves that typically exhaust to the discharge chimney, the chimney flow cross-section is usually larger than a radial duct used to cool the center body section of the rotor, in both the direction of slot width and along the longitudinal direction of the conductors. Since the cooling gas discharging through the chimney has already cooled and removed heat from the end section, the gas entering the chimney is at elevated temperature. The electrical conductor surrounding the chimney generates heat and also needs to be cooled, and this conductor temperature will be high because it is being cooled with gas at elevated temperature. This causes one of the hottest regions of the rotor to be near the location of the discharge chimney, which limits rotor output and electric power performance. At the same time, the large chimney flow area requires removing more electrical conducting area from the winding, causing increased electrical resistance and heating in the same area where the chimney is cooled with gas at elevated temperature. In addition, the discharge chimney will have less heat transfer surface area on its walls compared to the gas flow cross section in a typical radial cooling duct in the body section of the rotor. Furthermore, because of its large size, the discharge chimney is typically machined such as in a milling operation, and this leaves a smooth surface, and the resulting smooth wall further reduces heat transfer performance.
Accordingly, a need exists in the art for a discharge chimney having improved heat transfer characteristics to more effectively cool the end section of the rotor.